” . . so that they could maintain a system in which they denied some people freedom?”
Well, sort of. Slavery was the system that the U.S maintained before, during and for awhile, after the WBTS. But if the goal for the South had been to simple maintain that system for themselves they would have simply remained in the Union.
No, actually, without expansion of slavery into new states, slavery would have been extinguished politically. This is why a Republican President was unacceptable to the South.
Ah, but they couldn't protect slave imports or guarantee that slavery was expanded to the territories or that no state could outlaw slavery like the confederate constitution did. So it was half a loaf under the real Constitution or the whole loaf under the confederate one. Why is it surprising they chose rebellion?